Recently, there has been a problem that particulates contained in exhausted gas discharged from combustion engines of vehicles such as buses, trucks, construction machines and the like affect the environment and the human body.
There have been various ceramic filters which allow the exhausted gas to pass through porous ceramic, thereby capturing the particulates in the exhausted gas and purifying the exhausted gas.
As one example of such ceramic filters, there is used a honeycomb filter 30 in which a plurality of porous ceramic members 40 shown in FIG. 16 are bound by means of an adhesive layer 34 to constitute a column-shaped ceramic block 35, and a seal material layer 33 is formed around the column-shaped ceramic block 35. Moreover, as shown in FIG. 17, this porous ceramic member 40 is provided with a number of through holes 42 aligned in the longitudinal direction so that each partition wall 43 separating the through holes 42 from each other functions as a filter.
In other words, as shown in FIG. 17(b), with respect to each of the through holes 42 formed in the porous ceramic member 40, either of the ends on the inlet side or outlet side of the exhaust gas is sealed by a filling material 41 so that the exhaust
gas, flown into a through hole 42, is always allowed to flow out through another through hole 42 after having passed through this partition wall 43 that separates through holes 42; thus, when the exhaust gas passes through the partition wall 43, particulates thereof are captured by the partition wall 43 so that the exhaust gas is purified.
Moreover, a seal material layer 33 is formed on the outer circumferential portion so that one portion thereof is formed to prevent the exhaust gas from leaking from the through holes 42 exposed to the outside of the porous ceramic member 40.
With respect to a non-oxide-based ceramic material constituting the porous ceramic member 40 of this type, silicon carbide, which is excellent in heat resistance, and easily subjected to a recovering process and the like, is used in various vehicles such as large-size vehicles and vehicles having diesel engines.
Further, in addition to the above-mentioned particulates, the above-mentioned exhaust gas contains CO, NOx, HC, etc., and in order to remove these substances from the exhaust gas, an exhaust gas purifying catalyst converter, which has virtually the same shape as the above-mentioned honey comb filter 30 with a catalyst such as platinum deposited therein, has been proposed.
Moreover, in recent years, studies have been conducted on the next generation clean power sources which do not use petroleum as the power source, and among these, for example, fuel cells have been considered to be a very prospective power source.
The fuel cells, which utilize electricity that is obtained when hydrogen and oxygen react with each other to form water as a power source, have an arrangement in which oxygen is directly taken from the air while methanol, gasoline and the like are modified and utilized to provide hydrogen, and upon modifying these methanol, gasoline and the like, an exhaust gas purifying catalyst converter, which has virtually the same shape as the above-mentioned honey comb filter 30 with a copper-based catalyst deposited therein, has been utilized.
Generally, these honeycomb filter 30, an exhaust gas purifying catalyst converter, a catalyst converter for a fuel cell and the like are placed inside a cylinder-shaped metal shell, and used, and in this case, there is a gap between the honeycomb filter 30, the exhaust gas purifying catalyst converter or the catalyst converter for a fuel cell and the above-mentioned metal shell, and in order to fill the gap, a holding seal materials 50 shown in FIG. 18 is interpolated therein.
As shown in FIG. 18, the holding seal material 50 is provided with a convex fitting section 52 placed on one of the shorter sides of a base material portion 51 having a virtually rectangular shape, and a concave fitting section 53 placed on the other shorter side.
The convex fitting section 52 and the concave fitting section 53 are just fitted to each other when the holding seal material 50 is wound around the outer circumference of the honeycomb filter 30; thus, it is possible to prevent the holding seal material 50 from deviation.
Conventionally, the holding seal material of this type has been formed through the following first through fourth methods.
In other words, in the first method for manufacturing the above-mentioned holding seal material, first, a starting material containing an alumina source and a silica source is heated to approximately 2000° C., and subjected to a spinning process in a fused state, and then quickly cooled down to obtain ceramic fibers that has virtually the same alumina content and silica content. Then, a material is produced by aggregating the above-mentioned ceramic fibers into a mat shape. This material is stamped out by using a metal mold to manufacture holding seal materials.
In the second method for manufacturing the above-mentioned holding seal material, first, a spinning stock solution containing an alumina source and a silica source is prepared, and by discharging this solution through a nozzle, a precursor fiber having a true round shape in its cross-section is continuously obtained. Next, the long fiber of the precursor fiber obtained through the above-mentioned spinning process is sintered, and the resulting alumina-silica based fiber is then chopped into short fibers having a predetermined length. Next, the short fibers thus obtained are put into a mold to form a fiber aggregation having a mat shape. This fiber aggregation is stamped out by using a metal mold to manufacture holding seal materials.
Moreover, in the third method for manufacturing the above-mentioned holding seal material, a spinning stock solution, preliminarily prepared for use in an inorganic salt method, is supplied to a centrifugal nozzle, and the spinning stock solution is blown out of the nozzle by a centrifugal force exerted on the centrifugal nozzle to form precursor fibers. Next, the resulting precursor fibers are aggregated into a mat shape, and this mat-shaped aggregation is stamped out by using a metal mold to manufacture holding seal materials.
In the fourth method for manufacturing the above-mentioned holding seal material, first, an alumina fiber stock solution (alumina-silica fiber stock solution) is subjected to a spinning process to form a continuous long-fiber precursor, and an alumina long fiber is manufactured by sintering this continuous long-fiber precursor.
Next, after this alumina long fiber has been cut into alumina short fibers, these alumina short fibers are collected, untied, and laminated, and this is then pressed to form an alumina fiber aggregation having a mat shape.
Then, this mat-shaped aggregation is stamped out into a predetermined shape to manufacture holding seal materials.
The holding seal material, thus manufactured, is wound on the outer circumferential face of the above-mentioned honeycomb filter, the exhaust gas purifying catalyst converter or the catalyst converter for a fuel cell, and this is then housed in a metal shell; and in such a housed state, since the holding seal material is compressed in the thickness direction so that a repulsive force (face pressure) resisting against the compressing force is exerted in the holding seal material. The repulsive force thus exerted makes it possible to hold elements, such as the honeycomb filter, the exhaust gas purifying catalyst converter and the catalyst converter for a fuel cell, inside the above-mentioned metal shell.
In the case where the honeycomb filter, the exhaust gas purifying catalyst converter, the catalyst converter for a fuel cell, etc. are housed inside the above-mentioned metal shell through a press-fitting method, a metal cylinder member having an O-letter shape in its cross-section is used, and when these are housed inside thereof by using a canning method, a clam shell, which is formed by dividing a metal cylinder member having an O-letter shape in its cross-section into a plurality of pieces along the axis-line direction thereof, is used. Moreover, in addition to this method, a metal shell, which uses a tightening method in which welding, bonding and bolt-fastening processes are carried out by using a metal cylinder-shaped member having a C-letter shape or a U-letter shape in its cross-section, is also utilized.
However, with respect to the holding seal material manufactured through the first method, since this member is subjected to vibration and high temperatures of such as exhaust gas, when it is used, the face pressure is gradually lowered as time elapses, resulting in degradation in the holding property and sealing property of the catalyst carrier in a comparatively early period of time.
Moreover, with respect to the holding seal material manufactured by the first method, properties for securely holding the honeycomb filter, the exhaust gas purifying catalyst converter, the catalyst converter for a fuel cell, etc. for a long period of time are required; however, the conventional ceramic fibers, manufactured through the above-mentioned fusing method, has a very low level of crystallization rate (mullite rate), that is, less than 1% by weight, in addition to its high level of amorphous components. For this reason, when the resulting fibers are subjected to high temperatures for a long time, thermal shrinkage occurs as crystallization advances, resulting in brittleness in the fibers. Therefore, the holding seal material, manufactured by using these fibers, fails to provide a sufficiently high initial face pressure, and causes high degradation with time in the face pressure during the application.
In order to solve these problems, a method for increasing the crystallization rate of the ceramic fibers to approximately 10% by weight has been proposed; however, in this case, hardening of the fibers causes degradation in the elasticity and flexibility of the holding seal material and the subsequent degradation in the sealing property.
Moreover, with respect to the holding seal material manufactured by the second method, properties for securely holding the honeycomb filter, the exhaust gas purifying catalyst converter, the catalyst converter for a fuel cell, etc. for a long period of time are required; however, the alumina-silica based fiber having a round shape in its cross-section, manufactured in the second method, tends to lose its flexibility to become brittle, and is easily broken, when exposed to high temperatures for a long time. Therefore, the holding seal material manufactured by these fibers is susceptible to degradation with time in the face pressure.
Furthermore, with respect to the holding seal material manufactured by the third method, when the formation of ceramic fibers is carried out by using the blowing method, the basis weight (weight per unit area) of the mat-shaped aggregation comes to have a higher positional dependence.
In other words, the degree of aggregation in fibers is not constant with the result that when the position at which the mat-shaped aggregation is stamped out differs, the face pressure value of the resulting holding seal material tends to differ. Consequently, it has not been possible to obtain a holding seal material having excellent stability in quality.
Here, in the alumina fiber aggregation formed by the above-mentioned fourth method, alumina short fibers, used for the alumina fiber aggregation, fail to have sufficiently high mechanical strength, and have comparatively great dispersions, with the result that the initial face pressure of the alumina fiber aggregation becomes insufficient, and the degradation with time in the face pressure of the above-mentioned alumina fiber aggregation is comparatively large; therefore, there have been demands for improvements.
Here, “the initial face pressure” refers to a face pressure of an alumina fiber aggregation in a state where neither load nor heat is applied thereto.
The present invention has been devised to solve the above-mentioned problems, and an object of a first group of the present invention is to provide a holding seal material which has a high initial face pressure, and is less susceptible to degradation with time in the face pressure, to provide an alumina-silica based fiber excellent in mechanical strength and suitable for obtaining the above-mentioned holding seal material and a manufacturing method thereof, and also to provide a manufacturing method of alumina-silica based fibers capable of securely obtaining the above-mentioned alumina-silica based fiber excellent in mechanical strength easily.
Moreover, an object of a second group of the present invention is to provide a holding seal material which has a high initial face pressure, and is less susceptible to degradation with time in the face pressure, with excellent sealing properties, and a catalyst converter, and also to provide a manufacturing method of a holding seal material which is suitable for obtaining the above-mentioned holding seal material.
Furthermore, an object of a third group of the present invention is to provide a holding seal material which is less susceptible to degradation with time in the face pressure, and also to provide a manufacturing method of alumina-silica based fibers that are used for the above-mentioned holding seal material.
Furthermore, an object of a fourth group of the present invention is to provide a holding seal material excellent in quality stability, and also to provide a manufacturing method of a holding seal material which is suitable for obtaining the above-mentioned holding seal material.
Furthermore, an object of a fifth group of the present invention is to provide a holding seal material which is less susceptible to degradation with time in face pressure, and also to provide a manufacturing method of a holding seal material which is suitable for the above-mentioned holding seal material, a ceramic fiber aggregation and ceramic fibers thereof.
Furthermore, an object of a sixth group of the present invention is to provide a manufacturing method of an alumina fiber aggregation which has alumina short fibers having high strength with small dispersions so that it provides a sufficiently high initial face pressure, and is less susceptible to degradation with time.